The present invention relates to a reactor for autothermal reforming of hydrocarbons, including at least one reaction zone in which is arranged at least one catalyst for the reformation so that the educts involved in the reformation are converted while flowing through the reaction zone.
Such reactors are particularly suitable for applications with high demands on the rate of load changes as, for example, in fuel cell systems because they feature high dynamic response and good cold-start ability.
During the autothermal reforming of hydrocarbons, the fuel is reacted with atmospheric oxygen and water vapor to form a hydrogen-containing gas mixture. Apart from the endothermic reforming reactions of the hydrocarbons with water vapor,
                                              ⁢                                                                              C                  n                                ⁢                                  H                  m                                            +                              2                ⁢                n                ⁢                                                                  ⁢                                  H                  2                                ⁢                O                                      ←                          →                                                n                  ⁢                                                                          ⁢                                      CO                    2                                                  +                                                      (                                                                  m                        2                                            +                                              2                        ⁢                        n                                                              )                                    ⁢                                      H                    2                                                                                ⁢                                |                      ΔH            >            0                                                                                                      C                n                            ⁢                              H                m                                      +                          n              ⁢                                                          ⁢                              H                2                            ⁢              O                                ←                      →                                          n                ⁢                                                                  ⁢                CO                            +                                                (                                                            m                      2                                        +                    n                                    )                                ⁢                                  H                  2                                                                    ⁢                            |                  ΔH          >          0                    which proceed quasi-adiabatically and therefore involve a decrease in temperature, exothermic, so-called “partial oxidation reactions” occur in the process,
                                              ⁢                                                                              C                  n                                ⁢                                  H                  m                                            +                              n                ⁢                                                                  ⁢                                  O                  2                                                      →                                          n                ⁢                                                                  ⁢                                  CO                  2                                            +                                                m                  2                                ⁢                                  H                  2                                                              ⁢                                |                      ΔH            <            0                                                                                                      C                n                            ⁢                              H                m                                      +                          n              ⁢                                                          ⁢                              O                2                                              →                                    n              ⁢                                                          ⁢                              CO                2                                      +                                          m                2                            ⁢                              H                2                                                    ⁢                            |                  ΔH          <          0                    which at least partially compensate for the decrease in temperature by the endothermic reactions. Thus the thermal energy required for the endothermic reforming of the hydrocarbons can be provided by the exothermic partial oxidation of the hydrocarbons which takes place at the same time.
The complete oxidation of a hydrocarbon CnHm can generally be described as follows:
                    C        n            ⁢              H        m              +                  (                  n          +                      m            4                          )            ⁢              O        2              →                    n        ⁢                                  ⁢                  CO          2                    +                        m          2                ⁢                  H          2                ⁢        O        ⁢                                 ⁢        ΔH              <    0  
This complete oxidation is characterized by the fact that the so-called “excess-air factor” φ takes the value 1 (φ=1). Excess-air factor φ is defined as follows:
  ϕ  =            quantity      ⁢                          ⁢      of      ⁢                          ⁢      oxygen      ⁢                          ⁢      fed      ⁢                          ⁢      to      ⁢                          ⁢      the      ⁢                          ⁢      reaction              quantity      ⁢                          ⁢      of      ⁢                          ⁢      oxygen      ⁢                          ⁢      required      ⁢                          ⁢      for      ⁢                          ⁢      complete      ⁢                          ⁢      oxidation      
In autothermal reforming, oxygen is usually fed substoichiometrically. In this process, excess-air factor φ is typically in the range0.25<φ<0.35
At constant temperature, the H2- and CO-yields are theoretically higher at smaller excess-air factors. In practice, however, a lower temperature results at smaller excess-air factors which is why, in the limiting case, the H2- and CO-yields decrease again due to the lower reaction rate.
In the subsequent gas treatment, resulting unwanted CO is reacted with H2O in a water gas shift reaction to form CO2 and H2.CO+H2O←→CO2+H2|ΔH<0
Due to faster reaction kinetics, the exothermic oxidation reactions take place to a greater extent in the entry zone of the reactor, involving a marked increase in temperature in this region. The endothermic reforming reactions take place predominantly in the downstream reaction zone in which the temperature consequently decreases.
A sufficient reaction rate is crucial for the complete conversion of the hydrocarbons on the catalyst, the reaction rate depending on its rate constant k and the concentration of the educt components for a given chamber. Rate constant k is temperature-dependent, it being possible to describe the temperature dependence at least approximately by the Arrhenius equation
  k  =      A    ·          e              -                  Ea          RT                    (A=Arrhenius factor, Ea=activation energy, R=gas constant, T=temperature). In the entry zone of the reactor, the reaction rates of the hydrocarbons are usually sufficiently high due to the high educt concentrations and the fast oxidation reactions occurring there. The heat released by the oxidation reactions can produce temperatures of 900-1000° C. here. In contrast to this, the reaction rates in the exit zone of the reactor are relatively low which is attributable to the conversion-related reduced educt concentrations and to lower temperatures in this region.
At smaller excess-air factors as, for example, φ=0.25, the increase in temperature in the entry zone of the reactor is less pronounced. In this case, moreover, the temperature in the exit zone can decrease due to the endothermic reformation to such an extent that the high reaction rates required for complete conversion of the hydrocarbons are no longer reached. In this case, the H2-Yield decreases and residual hydrocarbons remain in the product gas.
The conversion of the residual hydrocarbons could be kinetically favored by injecting secondary air, that is, by increasing excess-air factor φ in the end region of the reactor. To this end, however, the secondary air would have to be compressed, involving an additional expenditure of not immediately available electric energy for a corresponding compressor.
This turns out to be problematic, especially also when using such reactors for autothermal reforming of hydrocarbons within the framework of pressurized fuel cell systems. Here, apart from the fuel cell air, the reformer educt air must also be compressed to system pressure through energy-consuming compression. The electric power consumption of the compressor required for this reduces the attainable efficiency of the fuel cell system.